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7.3 Energy Changes in Reactions
Reaction of sodium and chlorine
7.3 Energy Changes in Reactions
Chemical Bonds and Energy
What happens to chemical bonds during a
chemical reaction?
Chemical reactions involve the breaking of
chemical bonds in the reactants and the
formation of chemical bonds in the
products.
7.3 Energy Changes in Reactions
Chemical Bonds and Energy
The heat produced by a propane grill is a form of
energy.
When you write the chemical equation for the
combustion of propane, you can include “heat” on
the right side of the equation.
C3H8 + 5O2  3CO2 + 4H2O + Heat
7.3 Energy Changes in Reactions
Chemical Bonds and Energy
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
What happens to energy during a chemical
reaction?
During a chemical reaction, energy is either
released or absorbed.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
In an exothermic reaction, the chemical energy of
the reactants is greater than the chemical energy
of the products.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
In an exothermic reaction, the chemical energy of
the reactants is greater than the chemical energy
of the products.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
Endothermic Reactions
A chemical reaction that absorbs energy from
its surroundings is called an endothermic
reaction.
In an endothermic reaction, more energy is
required to break the bonds in the reactants
than is released by the formation of the
products.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
In an endothermic reaction, the energy of the
products is greater than the energy of the
reactants.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
In an endothermic reaction, the energy of the
products is greater than the energy of the
reactants.
7.3 Energy Changes in Reactions
Exothermic and Endothermic Reactions
At about 450°C, mercury(II) oxide decomposes
into oxygen gas and liquid mercury. The
decomposition of mercury(II) oxide is an
endothermic reaction.
2HgO + 181.7 kJ  2Hg + O2